Antioxidants such as mannitol, butylated hydroxytoluene and alpha-tocopherol enhance the growth of polyoma virus transformed and non-transformed BHK-21 cells. In the case of mannitol this is observed even in the absence of added calf serum. In part these effects may operate to protect cellular growth control mechanisms. On the other hand oxidants such as H2O2 and t-butyl hydroperoxide can inhibit growth and overall cellular protein synthesis, through mechanisms that are likely to involve radicals. In the case of H2O2 the inhibitory effects can nevertheless be reduced by 'prestressing' the cells with mild heat or with H2O2 itself. Paradoxically very low concentrations (10(-8) M) of H2O2 or t-butyl hydroperoxide can actually stimulate cell growth, even in the absence of serum. These stimulatory effects however do not appear to involve radicals as they are enhanced by inclusion of mannitol or DMSO in the medium.